1. Field of the Invention
The present invention relates to technical fields of small antennas and multiband antennas capable of being incorporated into a handheld device.
2. Related Art
In recent years, handheld devices such as cellular phones have become widespread, and demands are strong for miniaturization of the handheld devices. In particular, miniaturization of an antenna utilized by a handheld device is required, and techniques become important for providing a small antenna capable of being integrated into a handheld device. Although a planar antenna can be adopted as an antenna for a handheld device, a bandwidth strongly depends on the antenna size, and the size of the planar antenna is increased to support a wide band. Therefore, the miniaturization of the handheld devices is difficult. Accordingly, a wire antenna comprised of a linear conductor is generally adopted as an antenna for a handheld device. For example, as shown in FIG. 16, there is an example that linear patterns 101 having fold patterns are used as a monopole antenna. Such a wire antenna is suitable for miniaturization of the antenna itself.
However, in the example as shown in FIG. 16, in the case where the linear patterns 101 are disposed in an upper space on a circuit board 102 as a ground plate and the antenna is fed at a feeding point, it is necessary to reserve a distance from the circuit board 102 and metal parts to the linear patterns 101 to some extent. Therefore, the wire antenna shown in FIG. 16 needs wasteful spaces in the upper portion on the circuit board 102, and though the antenna itself is miniaturized, it is not suitable for use in an incorporated antenna in a handheld device.
In particular, a quarter-wave wire antenna functions as a dipole antenna as a whole by forming an image current on the ground plate. In this case, as the antenna is reduced in size, increased is contribution of radio wave radiated by the ground plate. Accordingly, when such an antenna is incorporated into a handheld device, holding the handheld device by hand directly affects the antenna, and antenna characteristics may deteriorate. Further, when a housing of the handheld device is a folder type, opening and closing the housing are equivalently changes in shape of the ground plate. Therefore, in an antenna incorporated into such a housing, antenna characteristics vary largely depending on whether the housing is opened or closed.
Further, when either the conventional planar antenna or wire antenna is used to constitute a multiband antenna allowing the use of a plurality of frequencies, the antenna size is large, it is difficult to adjust resonance frequencies to prescribed frequencies respectively, and it is difficult to ensure excellent antenna characteristics for all of the plurality of frequencies.